Soften the margarine (do not melt) and then using a handheld mixer combine with the all vegetable shortening. Blend in the vanilla extract. Cup by cup sift in the powdered sugar. Add soymilk if needed. You want the frosting the be thick enough to stand on its own, but not thick enough that you can't get it through a frosting tip. At this point split the batch in half. In one bowl add the cocoa powder and blend. You may need to add a touch more soymilk. Once both bowls of frosting are at the desired consistency prep your frosting bag. I used a closed star tip for these.

Consult the above pictures for the next steps. Using a spatula, spoon about a fourth of the vanilla frosting into the side of the bag. Make sure you get it on just one side. Now spoon roughly the same amount of chocolate frosting onto the other side of the bag. Now take some vanilla and spoon it on top of the chocolate and vice versa. Continue in this manner until all of your frosting is used up. Don't worry if it all gets a bit mixed together.

At this point you have two options. If you want your frosting to be more of a melange of chocolate and vanilla (like how mine look) gently squeeze the side of the frosting bag together. If you want it to be more of a separate look, close the top of the bag and get to piping!

I recommend doing at least one practice swirl because the first one will usually just have one type of frosting. Best of luck!